1. Field of the Invention
This invention relates to a sea velocity evaluator for accurately determining the velocity of radar echoes from waves in the sea in real time and, more particularly, to a method and apparatus for removing sea clutter from radar signals by continuously determining the sea velocity for a predetermined segment of a radar scan, and automatically selecting a filter centered at that velocity.
2.Description of the Related Art
Clutter is produced by unwanted radar echoes which clutter radar output and make the detection of targets difficult. It is well known that sea clutter should be filtered out of radar signals prior to analyzing the radar signals for the detection of targets. Both amplitude and Doppler spectrum of echoes received from the surface of the sea are dependent upon geographically and temporally variable parameters such as wave height, wind speed, direction of the waves relative to the radar beam, the presence of swells or waves, and the like. Nevertheless, since the sea is relatively uniform over areas of modest size, its average velocity (Doppler frequency) can be estimated. It is also known that once the average sea velocity is estimated, the sea clutter can be removed from the radar signals using a filter. Namely, a rejection filter can be centered on the average sea velocity to filter out the sea clutter from the radar signals.
Two techniques have been employed in the past to filter out sea clutter. Sea clutter could not be fully suppressed and target detection was hindered using these known techniques.
With the first technique, the radar system typically includes a rejection filter with a fixed rejection notch centered on a Doppler frequency of zero. Doppler frequency refers to a Doppler shift which corresponds to an amount of change in the observed frequency of an echo signal due to the relative motion of the radar system and the target. This rejection filter, together with a voltage-controlled local oscillator in a superheterodyne receiver which acts to keep the sea clutter within the rejection notch of the filter, attempts to suppress sea clutter by altering the echo frequency as the radar system scans in azimuth. This first technique treats echoes from all ranges of a radar scan commonly, thus reducing accuracy, because sea state is not constant over large geographic areas.
With the second technique, the radar system had to be physically disrupted periodically to make adjustments. The system used either a manual or an automatic selection of rejection filters from a few available choices using map zones having range and azimuth. The selection of the rejection filter was determined by whichever filter achieved the minimum total echo energy from all the echoes including targets as well as sea clutter. In effect, the rejection filter was selected on a trial and error basis in which the rejection filter which resulted in the minimum energy output or the fewest alarms was selected.
During the readjustment of the rejection filter in the second method, the radar output is cluttered and must be ignored. In other words, the radar system is out of commission during readjustment. As a result, the time period between readjustments had to be carefully selected to balance between too frequent disruptions in radar output and too slow response to changed sea conditions. Also, this second technique could not compensate for gradual change of the sea state occurring between readjustments of the rejection filter. The prior art techniques were not only inaccurate, but also produced erroneous measurements due to the presence of ships, rain, cellular storms, and the like. Further, both methods associated with the prior art could not accurately compensate for agile changes in carrier frequency because a change in carrier frequency causes a change in Doppler frequency. To partially compensate for this inability, the prior art techniques used filters with wide rejection bands. However, using filters with wide rejection bands reduces target detection by the radar systems associated with the prior art.